Radio diversity receiving system



July 6, 1954 c. w. EARP RADIO DIvERsIIY RECEIVING SYSTEM Filed Feb. 8, 1951 2 Sheets-Sheet l July 6,-'1954 c. w. EARP 2,533,213

RADIO DIVERSITY RECEIVING SYSTEM Filed Feb. 8, .1951 2 sheets-sheet 2 4 DETEC TIA/G UNIT "U l Inventor v CHAR/.E5 m EAP A Homey n Patented July 6, 1954 UNITED lS TATES PATENT O FfFfIfCE f 2,683,213 n RADIO DIVERSITY RECEIVING SYSTEM Charles William Earp, London, England, -ass'ignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware .Application February s, .1.951, serian No. 210,077

Claims priority, application Great Britain February 14, .1950

4 Claims.

This inventionirelat'es `to :radio 4diversity 4receivingrrsystems.

.More particularly, .it 'relates to"space-diversity radi'owreception of angularly mod-ulatedrc'arrier wives,l such `as for example;l frequency or phase modulated carrier waves.

'Space-'diversity reception of. amplitudeV modulated waves fis Well-known'andV this rmethod of Areception has been applied with 'success to short wave-'telephoneandfbroadcast vtransmissions. In i' space-diversity receiving systems in `general, two

or more-'separatel aerials are spaced far enough apart vat the receiving station, to yield signals Each having 'diierent fading characteristics'. said'faer-ial .is then :associated with a corresponding receiver or receiving channel tofprovidewits respective signal channel. Automatic gain control voltages may be combined and used to control the gain of all said receivers or receiving channels equally and the output from the detector stage in each said receiver'or receiving channel is fed into a common circuit to be combined therein. This common. circuit thus provides a single combined nal output or a single combined lowrfrequency output '('or channel) in receiving frequency or like angularly modulated waves (or signals), it is desirableand usual in practice, to apply limitation to the signal arnplitude prior to the .detector stage. If said amplitude limitation is imposed on each of the receivers or receiving channels 'in a space-diyers'ity receiving system forv said angularly modulated Waves, it is possible for a single channel, having alweak signal compared to the noise therein, to contribute-a substantially large Anoi-se voltage to the combined low frequency output. In other words, any limitation of the strongest channel or channels tends to exaggerate the effect of the noise from the remaining channel or channels, and variousdevices and arrangements have been suggested to -ensure more effective diversity reception of frequency modulated (FM) waves. Some of these suggestions while providing effective results under certain special conditions, tend to call for complicated design and operation but other such suggestions fail to provide, in practice, any very desirable results.

For example, a moderately eifectiveV though somewhat complicated system for diversity reception of frequency-shift telegraph signals, comprises a common frequency changing4 oscillator means to deriveoutputs (or channels) at the. same' intermediate frequency (I. F.) in each of three receivers. Two Aof these three I. F. outputs y'are then varranged -to be modified by displacement in frequency by means of -further frequency changing and ltering. The two I. vli'. outputs, so modined, 'are then combined with the remaining unmodified I. F'. -channel ina common limiter circuit. The limiter circuit exaggerates the differences in level between the three 4outputs so obtained. AThree filtersA are used following the said limiter circuit toselect-andseparate the three channels from each other.` Each of the channels, Vso separated, .is then further heterodyned to yan identicalj frequency and fed into its respective discriminator. Allv three discriminators in this case are .identical and they operate vinto acommon` output load circuit to provide Aa combined low frequency4 output (or channel). f

vSuch a system, though found satisfactory for the diversity reception of short-wave frequencyshift telegraph signals, is complicated in design as already'stated. Furthermore, inthe Case -of signals where the Yfrequency bandwidth in operation is comparatively large,.such as-forexample, in .FM radio linksor the like systems, Ia .plurality of oscillators as called .for in thesystem outlined above, will/tend .to producespurious beat frequencies thus endangering the eiiciency of the system.

It isY the primary object of the present invention toprovide an essentiallysimple space-diversity receiving system. A further object .is to provide such asystem which willibe satisfactory for all types of angularly modulated transmissions. According to one aspect of the ,presentinvention there isprovided a radio space-diversity .receiving system for the vreception of waves angularly modulated by .signals 4comprising a plurality of receiving antennae each coupled to a separate receiving `channel comprising means for vproducing 'ineach of said. channels angularly modulated carrier waves the mean frequency of all said modulated carrerwaves being identical and the instantaneous phases of said modulated carrier waves being substantially the. same, andmeans for combining said modulated carrier waves in a common circuit for detection. k

It will vbe Vseen thatV the invention enables the waves in all the channels to be directly combined.y Y

The production of 'the waves to bedirectly combined `may be carried out by convertingany frequency modulation -of wthe .incoming waves into aphasemodulation of a carrierandaccording toafurther aspect -the present 4invention provides a radio space-diversity yreceiving system forh the reception` of waves kangularly modulated by signals comprising a plurality of receiving antennae each coupled to a respective receiving channel, means for translating the waves in the respective receiving channels into corresponding cophasal carriers of a mean frequency the same for all the channels phase modulated by the said signals and means for combining said modulated carrier waves in a common circuit for detection.

The above-mentioned and other features and object of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a block diagram of one embodiment of a complete receiving system according to the present invention, and Fig. 2 is a schematic and block diagram in greater detail of some relevant parts of the embodiment as shown in Fig. 1.

In the embodiment according .to the present invention to be described hereinafter, use is made of a system employing two antennae, only by way of an example and to avoid complications in the description.

Referring now to Fig. l, I and 2 represent two antennae arrangements coupled via respective lines 3 and 4 to the receivers 5 and 6 respectively. The said receivers thus represent the sources of two signal waves of identical carrier frequency but having different fading characteristics as already stated above. A local oscillator 1 is coupled to both said receivers `5 and 6. From the respective mixer circuits in said receivers, the respective I. F. outputs are coupled via lines 8 and 9 to the two I. F. amplifiers I0 and I I respectively. Part of the outputs from I and II are fed via lines I2 and I3 to further respective frequency changers or mixers I4 and I5. It is clear that the said outputs from I0 and Il will be at the same frequency f, where f represents the unmodulated or center frequency. A common oscillator (preferably of a stable frequency) I6 operating at a frequency F (say), is coupled to said mixers I4 and I5 via lines I1 and I8 respectively. The outputs from the said mixers I4 and I5 are fed into filters I9 and 20 respectively. The filters I9 and 20 are similar in form and characteristics, and each of them may be of the band-pass type comprising, for example, two coupled tuned circuits. Moreover each said filter is tuned to the same beat frequency between said frequencies f and F, for example (F-f) or (F-I-f) in each case, and inherently imposes a small but substantially similar delay or phase shift on the signals selected. The output from the filter I9 is fed into a further frequency changer or mixer 2|, and part of the I. F. output from I0 is also fed directly via line 22 to said frequency changer 2I. Similarly the output from iilter 20 and part of the I. F. output from II via line 23 are fed into the further frequency changer or mixer 24. The output from each of said frequency changers 2I and 24 has a mean frequency F which, as already stated above, is also the frequency of the output from said common oscillator I6. The outputs from 2| and that from 24 will not only have the same frequency, as F in this case, but will also be substantially cophasal always, though the signals arriving at the respective inputs to said receivers and 6 may have any random phase relation thereinbetween.

The following is one possible theory or explanation of how this effect is produced. Assuming rst the simplest case where the incoming signal on both antennas is for a given instant unmodulated either in frequency of phase, and that due to the disposition of the antennas `with respect to the transmitter the signal received on antenna 2 has a phase ga with respect to the signal received on antenna I. This phase difference will carry through both intermediate frequency amplifiers I0 and II, and the output of amplifier I0 will be f+ ii0 (410 indicating that this is the reference phase) while the output of amplier II will be f+ {a.

Tracing the signal JI I0 received from ampliiier I0, in mixer I4 it will be combined with frequency F from oscillator I6 to become at the output of filter I5, plus the additional phase shift produced in the filter. Where the frequency (or phase) of the signal is not changing this additional phase shift will be substantially 90. Thus the output of filter I9 will be F--(fI- IIO)-I I90. In mixer 2I this output will be combined with the output of amplifier II), f+ 10 to produce an output F-I- 1:90.

Tracing the signal f| \,a from the output of amplifier II, at the output of mixer I5 it will be combined with frequency F, and at the output of filter 20 there will appear (again in the absence of modulation) F-(f-I Ia)} \,90 which when mixed in mixer 24 with the direct signal from amplifier II, f-I-{Ix will produce an output F+ i190- This output from mixer 24 is therefore cophasal with that from mixer 2I and is of course the same frequency.

Assume next that frequency modulation is added to the same signal. At the output of arnpliiiers I0 and Il we have respectively respectively where Af represents the frequency modulation. If the phase angles involved are ignored for a moment, it will be seen that at the output of filter I9 there appears F-(f-I-Af) which is combined in mixer 2I with fi-Af to produce an output of F. Likewise the output of mixer 24 consists of F. However, due to the frequency modulation the filters I9 and 20 no longer shift the output of F by 90 but by a different angle depending upon the modulation, this different angle being the same for both filters. Assuming this angle is at one instant, then the outputs of both mixer 2| and 24 will be which phase angle will be detected in the phase dernodulator 21. The frequency modulation of the original signals at aerials I and 2 has thus been translated into phase modulation at the respective output of said frequency changers 2I and 24.

Likewise in the case of phase modulation, the instantaneous change of phase is effectively equivalent to a change of frequency in that the filters I9 and 20 will shift the phase of energy passing therethrough by some angle differing from by an amount depending on the amount of phase modulation. These outputs from mixers massimi@ the oper-ation` of -said` demodulatorsori-detector circuit.'

Alternatively; f the outputefrom; said 'circuit' 2-5z cet-risfi'neh demodulatedor detectedlwithout'f refer ence tn sa/i'd-T oscillator |65: For thistpurpoiser the said output isfed into a frequency discriminator I circuitfof'any-known form"- (not' shown); theoutputk from:A said1V frequencydiscriminatoire representing; as Wehr-known; the rst'timedifferientialrof' theA modulation'envelope ofthe input` signals unless it= is subjected'- tofsuitable-Y compensating network.

Am particular `example of" ar' morex detailedcircuit arrangement for part of the system; d'escribedwith reference to" Fig; 1' Willnow be described; fbyjv way of?` illustration, in' conjunction withliig. 2': Thepart, nowt under consideration; is principally lthat `following f points A 'and Einliigz 1; andaswillbe'fseen; showsY onlythosev elements directly 'necessary-td the"v illustration of the.principles of the present invention, detailsl such. 'as-4 fior` example, power supplysources and connections' thereto and',y the like beingiornitted wherever possible: The; system as described aboveis one for converting frequency'modulatedv incoming signals to phase modulated signals'and has .the addedsignificantfeature. thati the signals at the output of said units l'andli (Fig. 1) have the same-frequency,--the-outputs from units 2| andlil,` (Fig. 1) are not-onlyl also of identical frequency f, (as E inthe apoya-description) but they are; cophasal as, We11. Thus. it is possible; to combine .thesesaid outputs always in phase Aini circuit.25.:.(Fig..1),.` It isevidentalso thatginstead` of only two signal. sourcesasshcwn in Fig. 1 above, any number of' separate signal sources may-be used and Y the outputs-from other-.units similar to units 2| and 24 (Fig: 1)--may also bei combined in phase in said circuit 25.

Referring now to Fig. 21, two signals at thel same frequency, such as for example from units I and respectively (Fig. 1) are introduced respectively at A and B. The A signal, at the output of unit l0 (Fig. 1) develops a voltage acros coil 29 which is coupled to a tuned circuit 3|) tuned to the frequency of the signal. A connection 3| feeds part of the voltage developed across 30 to the control (or first) grid of a mixer valve 32, shown here as a pentode by Way of an example only. An connection 33 feeds another part of the voltage developed across said circuit 3|) directly to the suppressor (or third) grid of a further mixer valve 34 also shown here as a pentode. Similarly the B signal, at the output of unit |I (Fig. 1) and at the same frequency as the A signal, is fed via coil 35 and tuned circuit 36 to the control grid of a mixer valve 31 and to the suppressor grid of a further mixer valve 38 via connections 39 and 43 respectively. The suppressor grid of each of the valves 32 and 31 is connected to a constant frequency'oscillator 4|, shown here as a triode valve V42 connected as a Hartley circuit, by Way of an example only. The anode of the mixer valve 32 is connected to the high tension supply (H. T.-{) via connections takin: Fig: 1r as |93 and z2 lirespectively ,..trnenahlezif -selection of thersamebeatbetweenftmiiequencgr:

- ofthe. valvesf 34andf13 8; are commcnedssintparalle 43gV tuned circuitsand?A connections, lillwl'dlerfrz theeanode offthefmixer valvezfiis: similarly-:con` nectedftothahiglr tension supplylH-.L'Iaerviasf connections146-aatuned circuit 411Sand connections. 48;: Thetunedf-circuits Miandi'M recoupled respectivesfurther tuned circuits .MsandSisaUdL- saidi-pairfoff tuned-:circuits=-.44;f;4l|sandr41 5B tuned to operatesas-'band-pass ltersetiteferredi;

ofy tli'eef oscillator. 4 landilthatsof' the: Aon'fBa signais,` both.. saidffsignals, as .alreadyustatedabeing;` atrthe same frequency:V Asmentioneclfaboue k,tides beats-ftlequency;=selectedi;wilL-ibei=the sametirneaclir.

A casca. .Thez-voltagesadeveloped acrnss:` the; circuitssfy 49,!and 5f|lfare fed'- intothe respective:controlsgrirlsi.k of-theefurtherrmixen-valves. 3.4 .fand-:i385 as shnwmr, andareftcombinedf-ormixedt thereinf.respiectiwlyz-V vvith.A the Ai. andtheB `vsig-:cialivoltages directlysf fediintm lthe. respective I suppressor grids:J ot'saidf valves-.viasconnectionsg33and ltasstaterhabovce. Itlisxevident ftronpth'e` descriptions that, .th'e'svolira agesrreaching: eachit of: said.y control fgridsff dilisis:fr inrfrequencyi from thosef reaching z itsirespectivea tion generated by said oscillatorfllilr An. outputt-r atftliis'difference'- frequency.; that issatitherfiequency1 of fthe oscillator.` 4.1;: is .selefzztedL in ith'e coils 5| coupled toa tuned-@circuit 523i As-fthetanodess the coil '5| is:v inV fact". the.` common; outputloadi.' andithetanodes are: connected to `tlre-.higln ten w sion supply (H;,T.`|) through saids coil Ita. importantfto notel at thisustage, that-so.' long .sa

Ythe signals.A and' B1 haveeidenticalr frequency;`

relativez phases therei'nbetweerrfhavernor-sensiblerv significance andi signals-z at: ther oscillation5k tre-.-v quency: derivedfacrosslcoil 511 fromivalves tanda 38-lmust: have1:subst'antially-k thefsame phase and Willi-thus additivelyr combine =.therein: Alsoztlfiatf: if' thetuned4 circuits #areftuned? exactly.'` torthetV middle-of thesignal-band andfffsaidfmiddl'e fre-..- quency y'is-being :receivedthenitheI voltages. dev ell.` oped across ooi15|Mby-fsaid'combinationwill` in exact phase quadrature with the voltages iu-rf...

jected from the oscillator 42 into. thelsuppresser f' grids: vof thei valves-v32-i'an'd''f31; This'.=` phase diifrencewof y is obtainedaf-ronr thesiinilar phase shift between theinput andL the-= outpuif-ofi'i the pair of tuned circuits 44, 49 and between the input and the output of the pair of tuned circuits 41, 50, each said pair being similarly lightly coupled.

As over the pass-band of said lters. the phase shift varies over 90 about a mean, when the signals at A and at B are frequency modulated so that similar frequency modulation is caused in said lters over the entire pass-band thereof, the resulting voltages in the anode circuits of said mixer valves 34 and 38, that is in coil 5|, will be phase modulated over :1 -90 about a mean phase which is in quadrature with that of the output from oscillator 4 I. The voltage, developed across the tuned circuit 52, coupled as stated to coil 5|, is fed to the control grid of a valve 53 (shown as a pentode by Way of example) arranged to operate as a limiter to remove therefrom any possible amplitude modulation present in the usual manner. The voltages as developed in the coil 54 in the anode circuit of said limiter valve 53 are then transferred by coupling as indicated by an arrow to the detectingy unit 55. Unit 55 may comprise, for example, a diiferential detector or a phase discriminator arrangement of any known form, in which the said volt- :anaemiav ages may be compared, in the usual manner, with the voltage from the said oscillator 4l used as a reference voltage. For this purpose the output of the oscillator 4l is transferred via a buffer amplifier 56 to a coil 51 as shown. A tuned circuit 58 tuned to the frequency of said oscillator 4| is coupled to said coil 5l and the voltage at the frequency of the oscillator 4I is thus transferred via connections 59, 60 to unit 55 to be utilised therein, as already stated.

As already referred to above, when the signals are tuned exactly, that is, when the tunable circuits are tuned exactly to the middle of the band of frequencies covered by the signals, voltages developed across 54 and 58 are in phase quadrature, so that the added and subtracted components fed to the differential detector, for example used in unit 55 will be equal. Under this condition the differential rectified output from unit 55, obtained at D will be Zero. When, however, signals A and B are frequency modulated, the output voltage developed across 54 is phase modulated as stated above and a differential output will be made available at D said output bearing a direct relation to said frequency modulations at A and B.

If a common automatic gain control arrangement is required with such a system, a suitable arrangement, as for example that shown following C, may be used. Here a connection 6| feeds part of the output voltage developed across coil 5I into a rectifier 62 via an isolating condenser 63 and a rectied voltage suitable for said control and dependent on the strength of the input to said rectifier 62 is made available at 64, the resistances B5, 66 and condenser 61 constituting a simple Well known load resistor and low pass lter arrangement for the purpose.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is V 1. A radio space-diversity receiving system for the reception of angularly modulated Waves comprising a plurality of receiving antennae, a plurality of receiving channels each coupled to a separate one of said antennae, two branches coupled in shunt in each channel, an oscillator common to all the channels, means for beating the waves in one branch of each channel With the output of said oscillator to produce a common predetermined beat frequency Wave, means for mixing the resultant waves with the Waves in said other branch, means for selecting in all the channels an output from said mixing means occupying the same portion of the frequency spectrum means in a given branch of each channel for translating the angular modulation of the signal Wave into phase modulation, and means for combining said outputs in a common circuit for detection.

2. A system according to claim 1 further including in a given branch of each channel coupled circuits tuned to the unmodulated frequency 0f the Waves passing therethrough and adapted to produce a phase shift of the waves passing therethrough which varies in accordance with the angular modulation of the Waves passing therethrough.

3. A system according to claim 2 further in- A cluding in a given branch of each channel means for producing a phase shift equal to in the mean frequency of the Waves passing therethrough and a shift that differs from 90 according to the angular modulation of the Waves passing therethrough.

4. A system according to claim 1 further including in said common circuit for detection means for differential detection of said combined output with reference to the frequency of the oscillator.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,941,070 Bailey Dec. 26, 1933 2,219,749 Oswald Oct. 29, 1940 2,448,866 Crosby Sept. 7, 1948 2,510,889 Hollingsworth June 6, 1950 OTHER REFERENCES Anderson, A Floating Beat Note, page 18 in QST, December 1926. Y 

